This invention relates to apparatus for monitoring the working stroke of a hydraulic ram, and in particular to monitoring apparatus for a hydraulic ram used for adjusting the cutting horizon of a mineral winning machine such as a coal plough.
In order to adjust the cutting horizon of a mineral winning machine such as a coal plough, control means of various types are known. With these known types of contorl means, the position of the plough guide, that is to say the longwall conveyor carrying the plough guide, can be tilted by means of hydraulic control rams in such a manner that the plough dips towards, or rises away from, the floor of the longwall working. In a known installation of this type, the control rams are arranged between a goaf-side conveyor attachment and the head pieces which attach the conveyor to the advance mechanisms of hydraulic roof support units positioned on the goaf side of the conveyor. In use, the control rams tilt the plough guide in the plane at right-angles to the floor of the working, there being a direct relationship between the working strokes of the rams and the angle of tilt. Where the conveyor is a scraper-chain conveyor, control rams are usually associated only with alternate channel sections (pans) of the conveyor. During the tilting movement, the plough guide pivots with the conveyor about the pivot joints of the head pieces. (Control means of this type is described in the specification of U.S. Pat. No. 4,045,089).
Difficulties are encountered with these known arrangements in controlling the extension of the individual control rams. This is particularly the case when the rams are arranged in groups of, for example, three rams, each groups being associated with a respective control valve arranged on the goaf side of the conveyor. From the position at which the control valves are operated, it is not possible to monitor, in a reliable manner, the working stroke of the individually connected control systems. In practice, this can lead to inaccurate tilting of the conveyor, and hence poor control of the cutting horizon of the plough. In particular, the entire installation may be tilted in the same direction, when differential tilting is required for different portions thereof.
In order to obviate these difficulties, it has been proposed (see British Patent Specification No. 2 113 305) to provide a common indicator for each group of hydraulic rams. Each ram has a cylinder and a piston movable relative thereto. The monitoring apparatus comprises an indicator which is connected to each of the rams by a respective transmission element. The apparatus is such that the indicator provides an indication of the position of each of the pistons relative to its cylinder. The transmission elements are constituted by Bowden cables which are positioned within, and protected by, the hydraulic hoses which are used to supply the rams with pressurised hydraulic fluid.
A particular disadvantage that has been observed with this monitoring apparatus, apart from the relatively short service life of the Bowden cables, is the poor elasticity of the hydraulic hoses. These hoses become reduced in length when subjected to pressure, and this leads to nonuniform monitoring at the indicator.
In order to eliminate the difficulties inherent in the use of Bowden cables, it has been proposed (see the specification of U.S. patent application Ser. No. 610,641--which is assigned to the assignee of the present application) to connect the indicator of a monitoring apparatus to the associated hydraulic ram(s) by a respective hydraulic line. Each hydraulic ram includes a metering cylinder having a metering chamber connected to the associated hydraulic line. The indicator comprises a cylinder housing a piston which is displaced against the pressure of a return spring in dependence upon the outward movement of the metering cylinder. The metering chamber(s) of the metering cylinder(s) and the cylinder chamber of the indicator form a closed system, i.e. sealed from atmospheric pressure, so that the movement of the indicator piston over a scale gives a visible indication of the extent to which the associated hydraulic ram(s) is (or are) extended.
The aim of the invention is to provide an improved form of monitoring apparatus of this type.